Conventionally, in order to damp and absorb vibrations transmitted from a vibration generating section, such as a linkage for supporting a load-carrying platform or the like, to a vibration receiving section, such as an axle or the like, there is an instance in which a vibration damping system is arranged between the vibration generating section and the vibration receiving section, wherein the vibration damping system is comprised of a laminated body including a plurality of hard plates having a rigidity, and a plurality of soft members having a viscoelasticity, with the hard plates and the soft members being alternately laminated with each other. When a vibration damping system including such a hybrid laminated body is arranged between the vibration generating section and the vibration receiving section, shocks or vibrations generated in the vibration generating section are damped and attenuated by the hybrid laminated body, thereby allowing lowering of the vibration level transmitted to the vibration receiving section.
Such a hybrid laminated body is generally designed so as to allow a relatively large deformation in the horizontal direction while supporting the weight of the vibration generating section. Thus, when the hybrid laminated body is subjected to a vibration while supporting the weight of the vibration generating section, i.e., while a positive pressure is being applied to the hybrid laminated body, the hybrid laminated body undergoes a shearing deformation primarily in the horizontal direction. On this occasion, however, because the lower end side of the vibration damping system is restrained by the vibration receiving section side, a so-called prying deformation occurs to the hybrid laminated body upon input of a vibration with a large amplitude. As a result of the prying deformation, one end of the hybrid laminated body in the laminating direction is applied with a compressive deformation and the other end is applied with a tensile deformation.
With increase in the amplitude of the input vibration, the degree of prying deformation occurring in the hybrid laminated body increases, and so does the degree of the tensile deformation of the laminated body. In this instance, because the soft member is made of a non-compressive material, and has a relatively large restrained surface as opposed to a small free surface area, there is a problem of isostatic stress concentration that occurs at the center portion of the restrained surface upon occurrence of the tensile deformation, thereby leading to a premature isostatic fracture even upon a relatively small tensile deformation.
Here, the meaning of the technical term “isostatic fracture” will be explained below. In ordinary vibration damping rubber which essentially does not include hard members, there is a relatively large free surface area. Thus, when a tensile force is applied to the rubber, based on its entering deformation into the inside of the external rubber portion, the rubber is allowed to undergo an elongation deformation without causing change in volume. In contrast, in the vibration damping system wherein a plurality of hard plates are embedded to have a large restraining surface area for the soft members made of rubber, as in the structure to which the present invention is applicable, when a tensile force is applied, the structure is allowed to deform in the region adjacent to the outer periphery, though the structure assumes a tensile state in its inside. In such a state, when the stress level, or the isostatic stress level, increases beyond a threshold level, there is an instance wherein the soft members due to its increased volume undergoes a fracture. This sort of phenomena is called as “isostatic fracture.”
There has been proposed, e.g., as disclosed in JP 2006-057833A, a vibration damping system including a hybrid laminated body wherein a plurality of hard plates having a rigidity, and a plurality of soft members having a viscoelasticity are alternately laminated with each other, a pair of flange members provided at both end portions of the hybrid laminated body in the laminating direction, and a displacement restriction member in the form of a chain accommodated in the hybrid laminated body for connecting the flange members with each other while causing a tensile force between the flange members s as to reduce the isostatic stress. In such a structure, the chain accommodated in the hybrid laminated body places the soft members under a pre-compressed state upon production of the vibration damping system so as to reduce the input level of the tensile load. Thus, as compared to the arrangement in which the displacement restriction member is not provided, it has been actually confirmed that the isostatic stress can be reduced to one half with respect to the predetermined degree of the prying deformation, and a significant improvement can be achieved in terms of the service life of the vibration damping system. It would be desirable, however, to make further improvements in the vibration damping system according to the proposal as explained above.